Abrupt spin transition in a modified-terpyridine cobalt(ii) complex with a highly-distorted [CoN6] core

The cobalt(ii) complex incorporating -conjugated substituent, [Co(Naph-C2-terpy)(2)](BF4)(2) (1; Naph-C2-terpy = 4-(2-naphthoxy(ethoxy))-2,2:6,2-terpyridine), exhibits an abrupt spin transition (ST) behavior (cooperative factor C = 0.91) while its solvated product, 12MeOH, shows gradual spin crossover (SCO) behavior (C = 0.49). Single crystal X-ray structural analyses demonstrated that the octahedral coordination core [CoN6] in 1 shows larger distortion in both high-spin and low-spin states than solvated 12MeOH or another two derivatives, [Co(R-terpy)(2)](BF4)(2) (R = 2-naphthyl (2), 9-anthracenyl (3)). The respective distortion parameters (sigma) are compared with those for previously reported SCO cobalt(ii) compounds. The highly-distorted [CoN6] core in 1 (sigma = 126 in the HS state and 101.6 in the LS state) was stabilized by strong intermolecular interactions and observed an abrupt ST behaviour.